1. Field of the Invention
This invention relates to an automatic transmission control system for an automotive vehicle.
Generally, an automatic transmission for an automotive vehicle is equipped with a torque converter and a transmission gear mechanism. The torque converter converts engine torque and transmits it to the transmission gear mechanism. The transmission gear mechanism transmits the torque to driving wheels and outputs reverse rotation to the driving wheels when it is in reverse gear.
The transmission gear mechanism, which typically includes a planetary gearset, has a plurality of frictional coupling elements, such as clutches, by which the turbine shaft and gears of the planetary gearset are selectively locked together and unlocked, and brakes, by which the gears are selectively fixed and released. These clutches and brakes are operated by means of a hydraulic control system or circuit so as to change a torque transmission path of the transmission gear mechanism, thereby shifting the automatic transmission.
2. Description of Related Art
When such an automatic transmission shifts up, for instance, from second gear to third gear, the hydraulic control system, on one hand, releases a 2-4 brake and, on the other hand, locks a 3-4 clutch and a coast clutch. The 2-4 brake, which is typically a band brake equipped with a servo piston, is fixed when hydraulic pressure is supplied into a fix or lock pressure chamber only and is released when a hydraulic pressure is supplied into a release or unlock pressure chamber and when a hydraulic pressure is released from both the lock pressure chamber and the unlock pressure chamber. Since a lock pressure is developed in the lock pressure chamber when the automatic transmission is in second gear, the hydraulic control system supplies an unlock pressure to the unlock pressure chamber and the 3-4 clutch for shifting up the automatic transmission from second gear to third gear in order to unlock the 2-4 brake and lock the 3-4 clutch.
If a hydraulic pressure is simply supplied to both the unlock pressure chamber of the 2-4 brake and the 3-4 clutch when an up-shift from second gear to third gear is needed, the transmission gear mechanism is subjected to shift shock and internal locking or double locking by conditions in which a hydraulic pressure is supplied to the 3-4 clutch and the 2-4 brake or by a timing at which the 3-4 clutch and the 2-4 brake are operated. In an attempt to eliminate such shift shocks and internal locking, a hydraulic control system of the automatic transmission is provided with a 2-3 timing valve for controlling an unlock pressure supplied to the unlock pressure chamber of the 2-4 brake in response to a hydraulic pressure supplied to the 3-4 clutch in an oil passage to the unlock pressure chamber of the 2-4 brake. Such a hydraulic control system is known from, for instance, Japanese Unexamined Patent Publication, No. 61-45,157. In an automatic transmission controlled by a hydraulic control system, having the 2-3 timing valve enables the 3-4 clutch and the 2-4 brake to operate at an appropriate timing, so as to prevent the automatic transmission from causing internal locking.
If a vehicle provided with an automatic transmission remains stopped for a long period of time, accidental inclusion of air into oil of the hydraulic control system may occur. If the air included in the oil stays in the releasing pressure chamber of the 2-4 brake, a delay in the development of an unlock pressure in the unlock pressure chamber may arise, due to the air inclusion, when an upshift from second gear to third gear takes place after restarting the vehicle. Consequently, the timing of operation between the 3-4 brake and the 2-4 brake lags. This leads to shift shocks and internal locking.
Some of the frictional coupling elements of the automatic transmission are provided with what is referred to here as a "drift-on-ball" mechanism, cooperating with either a piston of the friction element or a drum of the friction element, in order to prevent the coupling elements from operating accidentally. Such a drift-on-ball mechanism includes a bore, penetrating through either the piston or the drum, and a ball. The ball is seated on and unseated from a ball seat of the bore. While the frictional coupling element is locked up, the ball is urged to be seated on the seat by means of locking force acting on the piston or the drum so as to close the bore for assured piston operation. While the frictional coupling element is unlocked, the ball is unseated from the seat by means of centrifugal force, generated by rotation of the frictional coupling element, so as to permit hydraulic pressure acting on the piston to escape through the bore. Letting the hydraulic pressure acting on the piston to escape prevents an increase in hydraulic pressure, and hence an increase in locking force, acting on the piston while the frictional coupling element rotates at a high speed. Accidental lock-up of the frictional coupling element is eliminated, therefore.
However, when the drift-on-ball mechanism is installed in a first frictional coupling element, such as a 3-4 clutch, it has been found that there are some gear shift operations in which the 3-4 clutch can not be locked adequately. That is, when the automatic transmission shifts to a specific gear in which the 3-4 clutch must be locked, although a locking force acts on the piston of the drift-on-ball mechanism installed in the 3-4 clutch, it is necessary, in order to enable the ball of the drift-on-ball mechanism to operate, to seat the ball on the seat, overcoming the centrifugal force acting on the 3-4 clutch. For this reason, the larger that the centrifugal force acting on the 3-4 clutch is, the larger the locking force is required to be. However, a line pressure, acting as the locking force, is restricted to a certain maximum value by features of a hydraulic control circuit. This may prevent the ball from being seated on the seat. This, in turn, results in failure to maintain hydraulic pressure acting on the piston, so that the hydraulic pressure does not increase to a desired pressure which is necessary to displace the piston a predetermined distance which allows slippage of the 3-4 clutch. Consequently, the operational reliability of the 3-4 clutch is lowered.